Document the aural attribute and __aural__ preview mode

Summary:
Ref T4843. Document the new assistive features in the developer docs.

(Also use the recommended mode to set them. They're equivalent for aural=true (but not for aural=false), so this doesn't actually change anything.)

Test Plan: Read documentation.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4843

Differential Revision: https://secure.phabricator.com/D8926